FIELD OF THE INVENTION
[0001] The present invention relates to acoustic projectors, especially projectors for use
in low frequency military and civilian sonar systems, and in particular to underwater
flextensional projectors having improved stable performance with depth and linearity
with drive voltage level.
BACKGROUND TO THE INVENTION
[0002] Low frequency military and civilian sonar systems require compact, light weight,
high power, efficient, wide bandwidth acoustic projectors whose performance is stable
with depth and linear with drive voltage levels and which have a low manufacturing
and maintenance cost. Flextensional projectors are amongst the best ones presently
available to meet these requirements, one of the most promising flextensional projectors
being the barrel stave type. The barrel stave projector (BSP) is a compact, low frequency
underwater sound source which has applications in low frequency active (LFA) sonar
and in underwater communications. In one known BSP design, such as described in U.S.
Patent 4,922,470 by G. McMahon et al, a set of curved bars (staves) surround and enclose
a stack of axially poled piezo-electric rings. The staves act like a mechanical transformer
and help match the impedance of the transducer to the radiation impedance of the water.
Axial motion of the stave ends is transformed to a larger radial motion of the stave
midpoints. This increases the net volume velocity of the water, at the expense of
the applied force, and is essential for radiating effectively at low frequency.
[0003] This known BSP projector has slots between the staves which are required to reduce
the hoop stiffness and achieve a useful transformer ratio. However, these slots must
be waterproofed by a rubber membrane (boot) stretched tightly and glued with epoxy
around the projector. This boot also provides effective corrosion protection for the
Al staves. However, the variation in performance with depth of the BSP is suspected
to depend in part on the boot. At increasing depths, hydrostatic pressure pushes the
boot into the slots causing the shell to stiffen tangentially, increasing the resonance
frequency, and causing an increasing loss of performance. This depth sensitivity of
a barrel stave projector can be reduced somewhat by reinforcing the boot over the
slots. It is also possible to pressure compensate the BSP with compressed air or other
gas resulting in good acoustic performance at greater depths.
[0004] The slots in the BSP, as a secondary effect, provide a valuable nonlinearity in the
response of the projector to hydrostatic loading. The staves will deflect inwards
together under increasing hydrostatic loading (assuming no pressure compensation)
since the projector is air filled. Depending on the thickness and stiffness of the
rubber, it is reasonable to expect that as the slots close at great enough depths,
that closure of the slots due to increasing depth will force the boot back out of
the slots. The projector will now be very stiff and resistant to further effects of
depth until the crush depth of the now, effectively, solid shell is reached. This
provides a safety mechanism which may save the projector in case an uncompensated
BSP is accidentally submerged very deep or a pressure compensation system runs out
of air.
[0005] Variants of this known BSP have been built to optimise light weight, wide bandwidth,
low frequency, high power, and improved electroacoustic efficiency. Efficiency is
an especially critical parameter for the high power versions of the BSP because the
driver is well insulated from the water thermally. The boot's relatively poor thermal
conductivity contributes to the difficulty in cooling the BSP.
[0006] There is evidence that the interelement variability in performance amongst a set
of 20 of these projectors used in a horizontal line array was due largely to variability
in the boot's material properties. Most of these projectors subsequently failed due
to chemical incompatibility of the boots with the hydrocarbon-based towed-array fill
fluid, underscoring the need for consideration of chemical compatibility whenever
elastomer clad projectors are exposed to fluids other than seawater. The neoprene
boot is a potential weak point for the BSP in terms of damage due to rough handling.
Even a pinhole in the boot can lead to projector failure by flooding. Overhaul of
a barrel stave projector usually involves boot replacement. The cost of a custom molded
neoprene boot is approximately $20.00 but the labour cost of installing the boot is
typically several person hours spread over 2 days (of glue curing time) contributing
to the relatively high maintenance cost for these BSPs.
[0007] The inside surfaces of the (eight)staves of these BSPs are machined individually
from bar stock on a numerically controlled (NC) milling machine. The staves are then
mounted together on a fixture and the outside surfaces are turned on a tracer lathe.
The machining and handling costs are such that the staves are the most expensive parts
of the BSP. These BSPs are, as a result, both relatively costly to manufacture and
maintain.
[0008] Since the radiating surface of this BSP is waterproofed with a rubber membrane, it
is susceptible to chemical attack and degradation and damage due to flooding through
pinholes. The BSP suffers from variation of performance with depth caused by water
pressure forcing the rubber membrane into the slots between the vibrating staves of
the projector unless a pressure compensation system is fitted. The BSP shows nonlinearity
of performance versus drive voltage due to effects of the rubber membrane. Thus there
could be substantial advantages to accrue if it were possible to develop a one-piece
flextensional shell for the BSP that does not require a boot.
SUMMARY OF THE INVENTION
[0009] It is an object of the invention to provide an acoustic projector with reduced depth
sensitivity when submerged in water, improved efficiency and increased thermal conductance
to the surrounding fluid by the use of a one-piece thin walled folded shell as a radiation
surface.
[0010] An acoustic projector, according to one embodiment of the present invention, comprises
a pair of spaced apart end plates with an acoustic driver positioned between the end
plates, the driver having smaller cross-sectional dimensions than the end plates which
have edges secured to an outer one-piece thin walled shell that provides an enclosure
for said driver, the thin walled shell having a concavely inwardly bent surface between
the end plates and a plurality of axially extending corrugations to provide a predetermined
axial compliance and radial to axial transformation ratio.
[0011] An underwater acoustic projector, according to another embodiment of the invention,
comprises a pair of spaced apart end plates with an acoustic driver positioned between
the end plates, the driver having smaller cross-sectional dimensions than the end
plates which have outer edges secured to an outer one-piece thin walled shell that
provides a waterproof enclosure for said driver, the thin walled shell having a concavely
inwardly bent surface between the end plates and a plurality of axially extending
corrugations to provide a predetermined axial compliance and radial to axial transformation
ratio and wherein the shell is formed of a material selected from the group of ferrous
metals, non-ferrous metals, plastics or composites.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The invention will now be described in more detail with reference to the accompanying
drawings, in which:
Figure 1 is a perspective view of a known barrel-stave projector without a rubber
boot,
Figure 2 is a cross-sectional view along a longitudinal axis of Figure 1 with a rubber
boot in place but without the upper and lower end caps shown in Figure 1,
Figure 3 is a perspective view of one embodiment of a folded shell projector according
to the present invention with one fold removed to illustrate its interior,
Figure 4 is a view of one eighth of the outside surface of a folded shell showing
deformation and axial/radial transformer action resulting from the force applied by
an acoustic driver,
Figure 5 is a plot of a portion of the surface of a folded shell according to the
invention with a rounded cusp,
Figure 6 is a perspective view of a prototype folded shell plated onto an aluminum
mandrel after the outside contours have been machined but prior to dissolution of
the mandrel, and
Figure 7 is a perspective view of another embodiment of a folded shell projector according
to the present invention, a dual shell version with one quadrant cut away and one
end cap separated to illustrate the interior.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0013] Low frequency military and civilian sonar systems require compact, light weight,
high power, efficient, wide bandwidth acoustic projectors whose performance is stable
with depth and linear with drive voltage levels as well as being low in cost to manufacture
and maintain. Flextensional projectors are amongst the best ones presently available
to meet these requirements. One type of flextensional projector, known as the barrel
stave projector (BSP), is described in U.S. Patent 4,922,470 by G.W. McMahon et al.
This barrel stave projector, illustrated in Figures 1 and 2, contains a driver 1 formed
of a stack of axially poled piezo-electric ceramic rings and an enclosure formed by
a set of curved bars (staves) 2 with polygonal end plates 3. The staves 2 are secured
to flat sides of the octagonal end plates 3 with an adhesive (epoxy resin) and bolts
4 retained in threaded holes in the end plates. Caps 6 and 7 cover openings in end
plates 3.
[0014] Axial motion of the stave ends is transformed to a larger radial motion of the staves
midpoints. Slots 5 between the staves 2 are required to reduce the hoop stiffness
and achieve a useful transformer ratio. Those slots 5 must be waterproofed by a rubber
membrane (boot) that is stretched tightly around the projector and glued with epoxy.
This boot 8 (shown in Figure 2) is used for sealing purposes and may be formed of
a rubber membrane which, for variants designed for operation near 1kHz, is about 1
mm thick. It also provides corrosion protection for the Al staves used in these types
of BSPs.
[0015] The rubber membrane (boot) 8 which waterproofs the radiating surface of the BSP is,
however, susceptible to chemical attack and degradation with resulting damage due
to flooding through pinholes.
[0016] These BSPs also suffer from variation of performance with depth caused by water pressure
forcing the rubber membrane into the slots between the vibrating staves of the projector
unless a pressure compensation system is included. In addition, these BSPs exhibit
non-linearity of performance versus drive voltage due to the effects of that rubber
membrane.
[0017] The present invention provides a one-piece slotless flextensional shell for an underwater
acoustic projector which is inwardly concavely shaped similar to the BSP but which
does not require any boot. It is formed of a one-piece shell with no gaps or openings
in its outer surface. This shell achieves the required low hoop stiffness for low
frequency operation by using folds rather than slots as used in the BSP. This Folded
Shell Projector's (FSP) surface is formed of a thin-walled one-piece inwardly concavely
shaped shell containing corrugations (folds) running in the axial direction. The basic
concept of such a FSP is illustrated in Figure 3 with one fold removed to show the
inner piezoelectric driver 1'. The thin-walled folded shell 20 is inwardly concavely
shaped with a number of axially extending corrugations having valleys 22 and ridges
or cusps 24. The corrugations extend between end flanges 26 which are intended to
be connected to end caps 3'. Leads 23 extend from the piezoelectric driver 1' through
a central opening in one of the end caps 3'. Computer models of a slotless flextensional
shell indicated that if aluminum (A1) was used as the shell material, then a wall
thickness for practical designs would lie in the range of 1 to 2mm and that approximately
16 folds (corrugations) would provide the required performance. The depth of the corrugations
varies from a maximum at the center to zero at the flange. The axial dependence of
the depth of the fold is described by a polynomial of order N.
[0018] Figure 4 is a view of the outside surface of a folded shell derived from a computer
generated model showing deformation and axial/radial transformation action resulting
from the force applied by an acoustic driver. Valleys 22 and cusp 24 show the shell
20 in an undeformed state whereas 22d and 24d shown shell 20 when deformed. To avoid
the sharp cusps shown in Figure 4, a better termination for the fold's apex was considered
to be in a radius as illustrated at 24r in Figure 5. This change would eliminate sharp
edges on the outer wall which would have been hazardous to handle and easily damaged.
This change to the cusps of the corrugations would result in a modest increase in
shell mass and in the resonant frequency of the projector.
[0019] Low-cost high volume production of these thin-walled FSP shells would generally be
done by stamping a thin walled shell from non-ferrous or ferrous metals such as aluminum
or steel, or by molding or by casting in plastics or composites such as metal-matrix
or fiber-reinforced plastics. There are many suitable metals or other materials from
which a FSP may be manufactured with the best choices being ones that have low internal
acoustic damping, high stiffness, low density and which can be readily formed and
machined. A low cost version of a FSP could be made using injected molded thermosetting
fiber reinforced plastic but the acoustic damping of that material would reduce the
efficiency of the projector. This may be an acceptable trade-off for some applications.
Aluminum alloys have been used with great success in BSP and would be a suitable material
for forming a FSP. A protective coating on a metal FSP may be required for projectors
which are exposed directly to sea water for long periods of time. Those protective
coatings could be in the form of an anodised layer, an electroplated layer, paint,
etc. To construct a prototype FSP, however, electroforming was chosen as the most
economical method to produce the thin-walled shell. Other production methods (stamping
or molding) would have required the use of expensive dies and would not be practical
for manufacturing one of a kind prototype shells. The choice of electroforming metals
(Cr, Au, Ag, Cu, Ni) is rather limited and, of these, Ni was considered as a best
choice since it is corrosion resistant, has high stiffness, high strength and low
damping.
[0020] To manufacture the prototype Ni FSP, a numerically controlled (NC) mill was programmed
to make the required tooling which comprises a disposable hollow aluminum (A1) mandrel
(upon which the Ni is plated), disposable Al plate endcaps with Teflon™ gaskets (to
protect keyed ends of the mandrel from the plating process) and fixtures keyed to
the mandrel and to the dividing head of the mill to permit accurate registration before
and after plating. The
outside surface of an aluminum cylinder was NC milled to the contours of the desired
inside surface of the shell using standard ball nosed cutters to form the mandrel. That
inside surface is inwardly concavely shaped with 16 corrugations running in the axial directions.
The shell thickness was electroformed with Ni by plating onto the aluminum mandrel
which produces a perfect replica of that
outside surface to form the
inside surface of the prototype FSD. The outside surface of that plated mandrel, however,
is irregular at this stage. The plated mandrel was reinstalled in the NC mill and
the outside contours milled using chrome vanadium ball mills due tc the hardness of
the Ni electroformed shell. The bulk of the Al mandrel was then bored out on a lathe
with the resulting product, illustrated in Figure 6, having an Al hollow mandrel 30
and a Ni shell 20'. The remaining Al, the remains of the mandrel, was then dissolved
away in hot NaOH leaving only the Ni shell. That shell weighed 435.2 gm with a wall
thickness of 1.27 mm at the midpoints of the folds. The axial compliance of the shell
was measured to be 6.2 x 10
-9 ± 1.0 x 10
-9 m/N using a dial indicator to measure axial deflections.
[0021] The prototype FSP was completed using standard transducer construction techniques
by inserting a fiberglass wrapped stack of 10 parallel connected axially poled piezo
electric ceramic rings into the FSP prototype shell with two mild steel end plates,
two aluminum endcaps and four 3.2 mm diameter stainless steel stress rods being assembled
to complete the prototype. The ceramic rings have a smaller diameter than the minimum
diameter of the prototype shell. This type of assembly is shown and described in U.S.
Patent 4,922,470. The axially poled rings have a 50.8 mm o.d., a 38.1 mm i.d. and
10.1 mm thickness. The aluminum endcaps plug large access holes in the steel end plates.
In this FSP prototype, a cast epoxy gland was provided to waterproof the entry point
for electrical leads, and air fittings were included for a pressure compensation system.
[0022] The desired parameters for the FSP prototype shell which were originally selected
are listed below in Table 1.
Table 1
Geometrical Parameters |
Value |
N (axial fold depth exponent) |
4 |
n (number of folds) |
16 |
r1 (radius of flange) |
0.0399m |
Z0 (½ fold height) |
0.0511m |
R (radius of curvature of the inwardly concave surface of shells upon which the folds
are superimposed) |
0.30m |
w (shell wall thickness) |
0.00125m |
ao (fold depth) |
0.0075m |
flange (height) |
0.0132m |
mass |
435.2gm |
[0023] The wall thickness measured at midpoints of the folds was found to be 1.27mm (+.05mm
- .13mm). This agreed well with the selected desired wall thickness of 1.25mm. This
shell's axial compliance was measured by compressing it in a hydraulic press to apply
a known axial load. That measured value was

[0024] Table 2 summarizes the acoustic performance of the uncompensated prototype FSP obtained
from shallow water (30m depth) calibration and some preliminary trials'in deep water.
Table 2
Resonant frequency 2100 Hz |
TVR 123.8 dB re 1µPa/V @ 1m |
SL (@ 3kV) 193.4 dB re 1 µPa @ 1m |
Bandwidth 530 Hz |
Q 4.0 |
DI (at 2100 Hz) .98 |
G 24.5 µmho |
B 157.2 µmho |
Efficiency 65% |
Mass of complete projector 2.463 KG |
Figure of Merit 7.1 W/(Kg-kHz - Q) @ 3000V |
Depth Dependence of Resonant Frequency |
(uncompensated) 0.125 H
Z/m (in 50-250m depth range).
[0025] The following is a list of the symbols appearing in Table 2 with a brief explanation
as to what those symbols represent:
- TVR -
- Transmitting Voltage Response in units of decibels referenced to µ Pascal/Volt at
1 meter,
- SL -
- Source Level in units of decibels referenced to 1 µ Pascal at 1 meter,
- Q -
- a commonly used term for the dimensionless ratio of a resonance frequency to the bandwidth
of the resonance peak, where bandwidth is the frequency interval between the points
on the conductance versus frequency curve where the conductance has fallen to half
its peak value,
- DI -
- the Directivity Index measured in the x-z plane,
- G -
- the Conductance in units of µmho. Conductance is the real part of the admittance,
i.e. the real part of the ratio of the current through a device to the voltage across
it.
- B -
- the Susceptance in units of µmho. Susceptance is the imaginary part of the admittance.
[0026] The relatively high resonance frequency (compared to a nominal 1100Hz for a BSP)
reflects this prototype FSP transformer ratio and the shell compliance being lower
than the corresponding values for a BSP. Suitable modification to the geometrical
parameters can, however, reduce that resonant frequency. The TVR is equal to the best
available BSP but if the design frequency is reduced, the TVR would be expected to
decrease. The directivity was measured at resonance in two planes, the x-y and x-z
planes. The quoted efficiency of 65% was estimated using the directivity index (DI)
measured in the x-z plane, neglecting the effect of the smaller x-y plane directivity.
If the directivity had been integrated over all angles, the resulting efficiency would
be several percent higher.
[0027] Calibrations of this FSP were performed at drive levels ranging from 30-3000V and
the TVR was unaffected by the driver level over that range. This is in contrast to
the behaviour of BSPS which exhibit noticeable frequency shifts and TVR level changes
over this drive level range.
[0028] This FSP flextensional projector uses a one-piece thin walled metal shell as a radiating
surface and achieves low tangential stiffness by using folds rather than the staves
used in a BSP. This FSP one-piece shell is inherently watertight so that a rubber
boot is not required which leads to reduced depth sensitivity, improved efficiency,
increased thermal conductance to the surrounding fluid, higher reliability and better
interelement matching than present BSPs.
[0029] The prototype FSP was provided with a piezoelectric acoustic motor but other types
of drive motors could be employed in a FSP. A magnetostrictive drive motor, for instance,
could be fitted into the space where the piezoelectric stack resided in the previously
described prototype. Other types of acoustic drive motors that are suitable for use
in FSPs include electrostrictive drive motors based on material such as PMN (lead
metaniobate), electrodynamic drive motors (permanent magnet and coil) or hydroacoustic
motors.
[0030] The previously described prototype FSP contained 16 axially extending corrugations.
The number of corrugations could, however, be varied anywhere from 8 corrugations
upward to obtain optimum performance when different materials, wall thickness and
geometry are used to produce a folded shell. Various types of geometry would be suitable
for these types of FSPs. The radius of curvature R of the inwardly concave surface
of the shell upon which the folds are superimposed may be, for instance, 5 to 20 times
the radius of the flange and the maximum fold depth may be anywhere from 2 to 10 times
the thickness of the shell wall.
[0031] A variant of the known BSP described with respect to Figures 1 and 2 is described
in U.S. Patent 5,135,556 by R.J. Obara wherein the staves are shaped and arranged
to have a circular cross-section arrangement at the top and bottom of the BSP but
an elliptical cross-sectional arrangement midway the top and bottom. This forms a
projector that has a radius of curvature that varies continuously between fixed values
as the angle about the axis of the projector varies and which is alleged to provide
a wide bandwidth. Another variant of the known BSPs is a dual shell version developed
by Dennis F. Jones to provide an increased bandwidth. That dual shell BSP is described
by D.F. Jones and C.G. Reithmeier in an article entitled "The Acoustic Performance
of a Class III Barrel Projector" that was published in Proceedings of the 1996 Undersea
Defence Technology Conference and Exhibition, Nexus Media, Swanly, U.K. pages 103-108,
(1996). This dual shell BSP consists of 2 slightly different BSPs fastened together,
end to end, to create a single unit having a wide bandwidth. This dual shell concept
is also applicable to FSPs and one embodiment of a dual shell FSP is illustrated in
Figure 7 wherein one quadrant is cut away and one end cap is separated to illustrate
the interior.
[0032] The dual shell FSP illustrated in Figure 7 is formed by a bottom shell 50 and top
shell 60 which are joined together at flanges 58 and 68 secured to a central support
plate 54 of approximately twice the thickness of the end caps 53, 63. A piezoelectric
motor 51 is included inside shell 50 and a second piezoelectric motor 61 is included
in shell 60 with the central divider 54 being located between the two motors. An end
cap 53 hermetically seals the bottom of shell 50 while end cap 63 (shown separated)
is used to seal the top end of shell 60, the end caps having a larger diameter than
the piezoelectric motor. Electrical leads 65 for the motors extend through an opening
in end cap 63 where an epoxy gland (not shown) is utilized for waterproofing.
[0033] In the dual shell FSP illustrated in Figure 7, shell 50 and shell 60 are similar
in shape to the prototype FSP but differ slightly such that the lowest breathing mode
resonance frequencies are separated. When combined in the composite transducer, these
two separated modal responses result in a broad bandwidth. The difference between
the two shells 50 and 60 can be obtained by the shells having different lengths, wall
thicknesses, radii of curvature, fold depths or a combination of these differences.
Any one or combination of these parameters could be used to produce two separate resonances
in the TVR with a useful flat region between them. This flat region provides an increased
bandwidth over that which would be obtained from one of the shells.
[0034] The embodiments of the invention previously described all had identical folds or
corrugations in any one single shell. However, folds that are deeper than others with
different curvatures can be formed in a single shell in order to optimise performance.
These different folds could be alternated or one type of fold may be grouped on opposite
sides of the FSP and another type on the remaining sides. This later arrangement would
provide some directivity to the acoustic signal which emanates from a FSP.
[0035] The preferred embodiments of the FSP have been described as ones specifically directed
to underwater acoustic projectors but these FSPs can also be operated in air where
they can operate as low frequency loudspeakers in, for instance, an alarm system.
When a FSP is intended to be operated in the atmosphere, the one-piece shell will
protect the acoustic driver from dust particles or other types of air supported pollutants
which might exist in highly contaminated environments.
[0036] Several embodiments of the invention have been described but various modifications
may be made to the preferred embodiments without departing from the scope of the invention
as defined in the appended claims. Various manufacturing processes that could be used
to produce the folded shell for these FSPs at low cost include stamping, hydroforming,
rolling of metals and molding or casting of reinforced plastics or composites.
1. An acoustic projector comprising a pair of spaced apart end plates (3) with an acoustic
driver (1) positioned between the end plates (3), the driver having smaller cross-sectional
dimensions than the end plates (3), and the end plates having edges secured to an
outer enclosure for said driver:
CHARACTERIZED IN THAT said outer enclosure is a one-piece thin walled shell having
a concavely inwardly bent surface between the end plates (3) and plurality of axially
extending corrugations (22).
2. An acoustic projector as defined in claim 1, wherein the corrugations (22) have rounded
cusps (24).
3. An acoustic projector as defined in claim 1 or claim 2, wherein the thin walled shell
(20) has at least 8 axially extending corrugations (22).
4. An acoustic projector as defined in any preceding claim, wherein the thin walled shell
(20) has a flange (26) at each end which is secured to the end plates (3).
5. An acoustic projector as defined in claim 4, wherein the concavely inwardly bent surface
has a radius of curvature of about 5 to 20 times the radius of said flanges (26).
6. An acoustic projector as defined in claim 4 or claim 5, wherein the corrugations (22)
have a maximum depth at a midpoint along the length of the shell, which depth varies
axially and is zero at said flanges (26).
7. An acoustic projector as defined in any preceding claim wherein the corrugations (22)
have a maximum fold depth of about 2 to 10 times the shell wall's thickness.
8. An acoustic projector as defined in any of claims 1 to 6, wherein corrugations (22)
with at least two different maximum fold depths form said plurality of axially extending
corrugations.
9. An acoustic projector as defined in any preceding claim, wherein the shell (20) is
formed of metal that is 1 to 2 mm thick.
10. An acoustic projector as defined in claim 9, wherein the thin walled shell (20) is
formed of aluminum having an outer anodised protective layer.
11. An acoustic projector as defined in any preceding claim, wherein the driver (1) is
an electrodynamic driver motor, electrostrictive driver motor, hydroacoustic motor,
magnetostrictive driver motor or piezoelectric motor.
12. An acoustic projector as defined in claim 11, wherein the driver (1) is a piezoelectric
driver comprising a stack of parallel connected, axially poled ceramic rings.
13. An acoustic projector as defined in any preceding claim for use underwater, wherein
the walled shell (20) provides a waterproof outer enclosure and wherein the end plates
(3) have access openings through which electrical leads (23) to the driver (1) extend,
each opening being plugged by an end cap, with a waterproof gland sealing an entry
point for the electrical leads (23) through an end cap.
14. An acoustic projector comprising:
a central divider (54);
two acoustic drivers (51,61), each acoustic driver (51,61) being coupled between the
central divider (54) and a respective end plate (53,63), each acoustic driver (51,61)
having smaller cross-sectional dimensions than the end plates (53,63); and
two outer enclosures (50,60) for said drivers (51,61), each enclosure (50,60) extending
between said central divider (54) and a respective end plate (53,63) ;
CHARACTERIZED IN THAT:
each outer enclosure (50,60) is a one-piece thin walled shell having a concavely inwardly
bent surface between the central divider (54) and the respective end plate (53,63)
with a plurality of corrugations extending along the length of the shell; and
the two shells (50,60) have slightly different physical properties to provide slightly
different predetermined axial compliance and radial-to-axial transformation ratios.